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计算机网络专业译文关于IPv6
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原文:《Cisco Certified Network Associate Study Guide.Sixth Edition》的第 740-747 页
(作者:Todd Lammle 出版社:Wiley Publishing, Inc.)
英语原文:
I hope you're ready to learn about the nuts and bolts of Internet Protocol version 6 (IPv6),
because you're going to get the rub on it in this chapter!
You should have a solid hold on IPv4 by now, but if you think you could use a refresher, just
page back to Chapter 3, "Subnetting, Variable Length Subnet Masks (VLSMs), and
Troubleshooting TCP/IP." And if you're not crystal clear on the address problems inherent to IPv4,
you really should review Chapter 11, "Network Address Translation (NAT)”
People refer to IPv6 as "the next-generation Internet protocol," and it was originally created as
the answer to IPv4's inevitable, looming address-exhaustion crisis. Though you've probably heard
a thing or two about IPv6 already, it has been improved even further in the quest to bring us the
flexibility, efficiency, capability, and optimized functionality that can truly meet our ever
increasing needs. The capacity of its predecessor, IPv4, pales in comparison-and that's the reason
it will eventually fade into history completely.
The IPv6 header and address structure has been completely overhauled, and many of the features
that were basically just afterthoughts and addendums in IPv4 are now included as full-blown
standards in IPv6. It's seriously well equipped, poised, and ready to manage the mind-blowing
demands of the Internet to come.
I promise-really-to make this chapter pretty painless. In fact, you might even find your self
actually enjoying it-I definitely did! Because IPv6 is so complex yet elegant, innovative and
chock-full of features, it fascinates me like some weird combination of a brand-new Lamborghini
and a riveting futuristic novel. Hopefully you'll experience this chapter as the cool ride that I did
writing it!
Why Do We Need IPv6?
Well, the short answer is, because we need to communicate, and our current system isn't really
cutting it anymore-kind of like how the Pony Express can't compete with airmail. Just look at how
much time and effort we've invested in coming up with slick new ways to conserve bandwidth and
IP addresses. We've even come u p with Variable Length Subnet Masks (VLSMs) in our struggle
to overcome the worsening address drought It's reality-the number of people and devices that
connect to networks increases each and every day. That's not a bad thing at all-we're finding new
and exciting ways to communicate to more people all the time, and that's a good thing. In fact, it's
a basic human need. But the fore cast isn't exactly blue skies and sunshine because, as I alluded to
in this chapter's introduction, IPv4, upon which our ability to communicate is presently dependent,
is going to run out of addresses for us to use. IPv4 has only about 4.3 billion addresses available-in
theory, and we know that we don't even get to use all of those. There really are only about 250
million addresses that can be assigned to devices. Sure, the use of Classless Inter-Domain Routing
(CIDR) and Network Address Translation (NAT) has helped to extend the inevitable dearth of
addresses, but we will run out of them, and it's going to happen within a few years. China is barely
online, and we know there's a huge population of people and corporations there that surely want to
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be. There are a lot of reports that give us all kinds of numbers, but all you really need to think
about to convince yourself that I'm not just being an alarmist is the fact that there are about 6.5
billion people in the world today, and it's estimated that just over 10 percent of that population is
connected to the Internet-wow!
That statistic is basically screaming at us the ugly truth that based on IPv4's capacity, every
person can't even have a computer-let alone all the other devices we use with them. I have more
than one computer, and it's pretty likely you do too. And I'm not even including in the mix phones,
laptops, game consoles, fax machines, routers, switches, and a mother lode of other devices we
use every day! So I think I've made it pretty clear that we've got to do something before we run
out of addresses and lose the ability to connect with each other as we know it. And that
"something" just happens to be implementing IPv6.
The Benefits and Uses of IPv6
So what's so fabulous about IPv6? Is it really the answer to our coming dilemma? Is it really worth
it to upgrade from IPv4? All good questions-you may even think of a few more. Of course, there's
going to be that group of people with the time-tested and well-known "resistance to change
syndrome," but don't listen to them. If we had done that years ago, we'd still be waiting weeks,
even months for our mail to arrive via horseback. Instead, just know that the answer is a
resounding YES! Not only does IPv6 give us lots of addresses (3.4 x 10
38
=definitely enough), but
there are many other features built into this version that make it well worth the cost, time, and
effort required to migrate to it. Later in the chapter I'll talk about all that effort in the section called
"Migrating to IPv6." In it, I'll cover some of the transition types required to move from version 4
to version 6, and I promise you'll discover that the huge benefits of migrating will vastly outweigh
any associated cons.
Today's networks, as well as the Internet, have a ton of unforeseen requirements that simply were
not considerations when IPv4 was created. We've tried to compensate with a collection of add-ons
that can actually make implementing them more difficult than they would be if they were required
by a standard. By default, IPv6 has improved upon and included many of those features as
standard and mandatory. One of these sweet new standards is IPSec-a feature that provides end-to-
end security that I'll cover ili Chapter 14, "Wide Area Networks." Another little beauty is known
as mobility, and as its name suggests, it allows a device to roam from one network to another
without dropping connections.
But it's the efficiency features that are really going to rock the house! For starters, the header in
an IPv6 packet have half the fields, and they are aligned to 64 bits, which gives us some seriously
souped-up processing speed-compared to IPv4, lookups happen at light speed! Most of the
information that used to be bound into the IPv4 header was taken out, and now you can choose to
put it, or parts of it, back into the header in the form of optional extension headers that follow the
basic header fields.
And of course there's that whole new universe of addresses (3.4 x 10
38
) we talked about already.
But where did we get them? Did that Criss Angel-Mindfreak dude just show up and, Blammo? I
mean, that huge proliferation of addresses had to come from somewhere! Well it just so happens
that IPv6 gives us a substantially larger address space, meaning the address is a whole lot bigger-
four times bigger as a matter of fact! An IPv6 address is actually 128 bits in length, and no
worries-I'm going to break down the address piece by piece and show you exactly what it looks
like coming up in the section "IPv6 Addressing and Expressions." For now, let me just say that all
2
that additional room permits more levels of hierarchy inside the address space and a more flexible
address architecture. It also makes routing much more efficient and scalable because the addresses
can be aggregated a lot more effectively. And IPv6 also allows multiple addresses for hosts and
networks. This is especially important for enterprises jonesing for availability. Plus, the new
version of IP now includes an expanded use of multicast communication (one device sending to
many hosts or to a select group), which will also join in to boost efficiency on networks because
communications will be more specific.
IPv4 uses broadcasts very prolifically, causing a bunch of problems, the worst of which is of
course the dreaded broadcast storm-an uncontrolled deluge of forwarded broadcast traffic that can
bring an entire network to its knees and devour every last bit of bandwidth. Another nasty thing
about broadcast traffic is that it interrupts each and every device on the network. When a broadcast
is sent out, every machine has to stop what it's doing and respond to the traffic whether the
broadcast is meant for it or not.
But smile everyone: There is no such thing as a broadcast in IPv6 because it uses multicast traffic
instead. And there are two other types of communication as well: unicast, which is the same as it is
in IPv4, and a new type called anycast. Anycast communication allows the same address to be
placed on more than one device so that when traffic is sent to one device addressed in this way, it
is routed to the nearest host that shares the same address. This is just the beginning-we'll get more
into the various types of communication in the section called "Address Types”
IPv6 Addressing and Expressions
Just as understanding how IP addresses are structured and used is critical with IPv4 addressing, it's
also vital when it comes to IPv6. You've already read about the fact that at 128 bits, an IPv6
address is much larger than an IPv4 address. Because of this, as well as the new ways the
addresses manage. But can be used, you've probably guessed that IPv6 will be more complicated
to no worries! As I said, I'll break down the basics and show you what the address looks like, how
you can write it, and what many of its common uses are. It's going to be a little weird at first, but
before you know it, you'll have it nailed!
So let's take a look at Figure 13.1, which has a sample IPv6 address broken down into sections.
FIGURE 13 .1 IPv6 address example
So as you can now see, the address is truly much larger-but what else is different? Well, first,
notice that it has eight groups of numbers instead of four and also that those groups are separated
by colons instead of periods. And hey wait a second…there are letters in that address! Yep, the
address is expressed in hexadecimal just like a MAC address is, so you could say this address has
eight 16-bit hexadecimal colon-delimited blocks. That's already quite a mouthful, and you
probably haven't even tried to say the address out loud yet!
One other thing I want to point out is for when you set up your test network to play with IPv6,
because I know you're going to want to do that. When you use a web browser to make an HTTP
connection to an IPv6 device, you have to type the address into the browser with brackets around
the literal address. Why? Well, a colon is already being used by the browser for specifying a port
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